1. Field of the Invention
This invention relates to a method for making investment molds having a barrier layer at the mold-metal interface for the casting and directional solidification of superalloys.
2. Background of the Invention
Shell molds for precision casting of steel and superalloy parts are, in general, composed of refractory oxide particles bonded together by a silica or phosphate gel. Such molds are generally formed by the "lost wax" process wherein a wax pattern is immersed repeatedly in a liquid slurry of the refractory oxide particles in a silica- or phosphate-bearing binder. Sufficient time is provided between immersions to allow the slurry coat to partially or completely dry on the wax. After a sufficient thickness of ceramic has built up on the wax, the wax is removed by chemical dissolution or melting in a steam autoclave or in a furnace. The mold is then fired, typically at 1,000.degree.C for 1 hour, to give it sufficient strength to withstand the casting process.
Chemical reactions between the mold and the cast metal are a minor problem in conventional casting due to relatively low temperatures and short times that the mold is in contact with molten metal. However, for the plane front solidification of eutectic superalloys, severe metal-mold reactions frequently occur. These are due to the long contact time (up to 30 hours) of molten metal with the mold, the high temperatures (.about.1,800.degree.C) required in the casting process to enable high growth rates during solidification, and the high concentration of reactive elements in the superalloy such as carbon, aluminum, and titanium. In particular, attempts to cast tantalum carbide-reinforced, eutectic superalloys with high nickel content in standard shell molds results in such a severe loss of carbon that the tantalum carbide reinforcing phase is absent from the final cast microstructure, producing a useless casting.
The mold-associated cause of this reaction is the silica phase (5-15 Wt%) present in the shell mold. Silica has a small negative free energy of formation and is reduced by the reactive elements in the eutectic super-alloys.
With reference to FIG. 1, when NiTaC-13, a monocarbide reinforced superalloy, is cast in mold 10, an example of the prior art, a reaction occurs between the cast metal and the silica phase of the mold. The result of this reaction is the bright phase denoted by reference numeral 12 which is NiTaC-13 metal penetrated into the mold and reacted with the silica phase of the mold. The resultant casting is defective due to decarburization of the cast alloy and because of poor surface finish.
Other prominent features in FIG. 1 include plastic mounting media 14, coarse backup grains 16 of alumina from the fluidized bed employed for the application of a sand coat between layers of mold materials, pores or voids 18 in the mold structure which result because of material pullout during polishing, or an actual void in the mold structure, and undissolved alumina 20 of one of the flours comprising the material composition.
Other features shown are mullite 22, light gray in color, a silica rich liquid phase 24 in the mullite 22, dark gray in color, and small grains 26 of flour of alumina material of the face coat. There is no barrier layer present at what may be termed the interface between the mold and the cast metal.
An object of this invention is to provide a new and improved process for making molds for the casting and directional solidification of superalloys therein.
Another object of this invention is to provide a new and improved process for the casting and directional solidification of superalloys.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.